This invention relates generally to inductance elements and particularly to planar inductors deposited on an electrically insulating substrate.
The word spiral as used herein is intended to encompass a broad class of structures which exhibit a clockwise or counterclockwise outwardly winding path beginning in a substantially centralized location in which each winding is successively longer than the previous winding. This definition is intended to embody generally rectangular, polygonal, oval, elliptical, and circular spirals as well as other irregular yet generally spiraling shapes. For illustrative purposes, generally circular spirals are shown in most of the figures.
Electrical inductance elements commonly comprise three-dimensional wire coils which are cylindrically or helically wound. Mutual inductance between adjacent wire segments containing current flowing in the same direction causes an inductive effect. Thus, arranging a conductor in a wound coil configuration produces an inductance because of the mutual parallel arrangement of the wire segments carrying the current. Problems are encountered in utilizing such known helically wound wire inductance coils in a printed circuit board or on a hybrid substrate because of the excessive three-dimensional area or space required by the wire wound coil. The limited space available for utilization by a printed circuit board often can actually prohibit the use of helically wound wire inductor coils.
Mere flat or planar spirally printed circuit elememts also are known and can achieve small inductance values. A planar spiral configuration is printed on one side of a substrate and can provide various values of inductance depending upon the geometry of the convoluted spiral. However, the use of such single planar spiral patterns itself is often limited by two-dimensional space limitations. It would be desirable to provide a planar spiral inductance element which achieves a maximum of mutual inductance and, therefore, a higher net inductance than a single spiral pattern for a given substrate area.
In addition, although methods are known for trimming three-dimensional wire inductor coils to vary the inductance values thereof, no efficient method is known for trimming or otherwise adjusting the value of flat or planar printed inductance elements.
An object of the present invention is to provide a new planar spiral inductance element which achieves a maximum of mutual inductance.
Another object of the present invention is to provide a new planar inductance element which achieves increased values of inductance for given two-dimensional areas.
A further object of the invention is to provide a two terminal planar inductance element which can be utilized in hybrid circuits on insulating substrates.
Still another object of the present invention is to provide a planar inductance element which is trimmable to provide variable values of inductance.
In the exemplary embodiment of the invention, a planar inductor is disclosed for use in electronic circuit apparatus or hybrid circuits on insulating substrates. A first substantially flat spiral coil is disposed on one side of the substrate and has an inner end and an outer end. A second substantially flat spiral coil is disposed on the opposite side of the substrate and also has an inner end and an outer end. The coils are spiraled in the same clock direction as viewed from one side of the substrate with one coil spiraling in and the other spiraling out. The inner ends of the coils are joined through the substrate to couple the coils in series. The outer ends of the coils provide terminals for the inductor, preferably leading away from the respective coils in generally diametrically opposite directions.
A novel feature of the invention is the provision of means for trimming the coils to vary the inductance thereof. In one form of the invention, the trimming means comprises substantially radially disposed conductive paths joining and shorting at least some of the spiral paths of one or both of the flat spiral coils. The conductive paths may be discretely and/or sequentially cut between the spiral paths to vary the inductance value thereof.
In another embodiment, the trimming means comprises a conductive layer of very low resistive material deposited on the substrate beneath one or both of the coils and shorting the spiral paths. This results in a continuously increasing inductance value depending upon the extent of trimming or severing of the conductive layer to electrically isolate a portion of the spiral path.
In a third embodiment, the trimming means comprises a conductive layer deposited on the insulating substrate. A pattern having inductive characteristics is cut in this conductive layer preferrably by laser.
Other objects, features and advantages of the invention will be apparent from the following detailed description taken in connection with the accompanying drawings.